Where does Blender "connect" to NVIDIA drivers?
It was a CUDA issue.  Blender needs nvidia-cuda-toolkit to find &amp; connect your GPU. The Debian Wiki directions for installing CUDA didn't work
for me, so I needed to download the most recent CUDA version and manually install it.
To start from scratch:

Uninstall NVIDIA &amp; CUDA:
$ apt purge nvidia*
$ apt purge cuda*
Download proper NVIDIA drivers
Download proper CUDA toolkit for your system (I used deb[local])
Before installing drivers, you must obtain the proper kernel headers for the NVIDIA driver to build with.From Debian Wiki:
$ apt install -t buster-backports linux-headers-amd64
$ apt install -t buster-backports linux-image-amd64  #installs the kernel from backports to match
Now it gets tricky: Disable the default nouveau driver
$ echo blacklist nouveau &gt; /etc/modprobe.d/blacklist-nvidia-nouveau.conf
Reboot to multi-user runlevel. #This will disable the GUI user after reboot
$ systemctl set-default multi-user.target 
$ systemctl reboot


NOTE WELL: When you restart your computer, your screen won't work. Everything you do will be from the command line.


From command line (or Safe Mode), navigate to directory with NVIDIA run file:
$ bash NVIDIA-Linux-x86_64-your.version#.run
During installation, you'll be asked questions, say YES to everything and install NVIDIA driver. After reboot, SCREEN SHOULD START UP USING YOUR GRAPHICS CARD.
Remind your computer to use the GUI every time it starts up:
$ systemctl set-default graphical.target


NVIDIA is good to go, but Blender still won't recognize your GPU because CUDA needs to be downloaded with nvidia-cuda-toolkit.
 


Download/Install CUDA Debian Installer from CUDA docs remembering to modify the PATH and LD_LIBRARY_PATH variables.
Make sure FILEPATH has been changed correctly.
From Debian Wiki, install nvidia-cuda-toolkit picking up any missing pieces:
$ apt install nvidia-cuda-dev nvidia-cuda-toolkit
Update everything with $ sudo apt update
Reboot computer: $ systemctl reboot
Open Blender, check Edit --&gt; Preferences --&gt; System, and GPU's alive and well